Various types of surgical procedures now require objects, such as tools or tracking devices, to be temporarily secured to a bone during a procedure. For example, during image guided surgery, it is sometimes necessary to attach active or passive trackers to bones in order to determine the location of a bone with respect to the image. This is required, for instance, for alignment procedures, such as fracture reductions. It is also often necessary to attach active or passive dynamic reference objects to bones. In addition, it is sometimes necessary to temporarily attach objects to bones, such as drill guides or templates to perform a subsequent drilling or sawing operation.
Furthermore, it is sometimes necessary to secure objects to the bone which must remain temporarily secured after the initial surgical procedure. For example, external fixation devices are sometimes secured to bones or bone fragments to hold the bone and bone fragments in place during healing. Also, spinal rods must sometimes be secured to vertebrae, thereby temporarily mechanically immobilizing areas of the spine, to treat certain types of spinal column disorders. Also, to analyze and treat certain joint pathologies, it is sometimes necessary to attach trackers to bones to measure the discrete joint movements, such as, for example, to measure the motion patterns of the human knee.
It has been known in the past to use bone screws to secure objects to bones. Bone screws generally have a coarse thread at one end for engaging the bone and a means at the other end to easily attach clamps or instruments thereto. While bone screws have worked well in the past, they are susceptible to rotational displacement which can either loosen or tighten the bone screw. This rotational displacement can be caused in different ways, such as by the normal cyclical force applied to the bone screw or through inadvertent contact with the object attached to the bone screw by an external source.
It is clear that rotational displacement of the bone screw will affect the object secured to the bone screw. For example, any rotational displacement of the bone screw may cause a rotational displacement of the object with respect to the bone. Furthermore, any rotational displacement of the bone screw may cause an axial displacement of the bone screw along the axis of rotation of the bone screw with respect to the bone. It is clear that any axial and/or rotational displacement of the bone screw with respect to the bone will also cause an axial and/or rotational displacement of the object with respect to the bone.
In some cases, any rotational or axial displacement of the screw can have a significant effect on the procedure. For example, during image guided surgery, the objects, such as tracking devices or instruments, secured to the bone screw will be registered in six degrees of freedom, namely three displacement degrees of freedom and three rotational degrees of freedom. Accordingly, any axial or rotational displacement of the bone screw will cause an axial or rotational displacement of the object and require re-registration of the object secured to the bone screw.
In the past, in order to avoid rotational displacement of the bone screw, and therefore rotational or axial displacement of objects attached to bone screws, more than one bone screw has been used to attach an object to a bone. For example, two or more bone screws, displaced along the bone, have been used to prevent detrimental effects caused by rotational displacement of one bone screw. However, using additional bone screws weakens the bones being treated and increases the length of time of the procedure. In addition, in some cases, it is not practical to use several bone screws because the bones or bone fragments are simply not large enough. Also, use of several bone screws restricts access to the operative site, which can be the case with the vertebrae of the spine.
Accordingly, there is a need in the art for a device which prevents rotational displacement of bone screws. In addition, there is a need in the art for a device which better axially and rotationally secures objects to bones.